1. Field
The field of the present invention relates to lithograph technology for fine image fabrication, and in particular to a lithography simulation system and measurement of critical dimensions.
2. Description of Related Art
Fine image fabrication by lithography uses technologies like particle beam writers for writing fine images on a plate coated by a particle beam sensitive resist and an optical projection lithography method that uses a mask having transparent and opaque parts on the surface of the mask for generating a fine image on a plate coated by a photo sensitive resist. An example of particle beam writers is an electron beam writer that is used for writing fine images on both silicon wafers and masks for optical projection lithography. A technology that uses electron beam writers for making fine images for semiconductor integrated circuits directly on the semiconductor wafer is called Electron Beam Direct Writing (EBDW) technology. EBDW technology is suitable for integrated circuit fabrication that requires a quick turn around time.
A fundamental problem with conventional lithograph technologies is image quality degradation and resolution limits caused by chemical and physical effects in the process of the technologies. An example of such phenomena is proximity effect, which occurs in both electron beam writing and optical projection lithography and causes differences between intended patterns and images obtained by the lithography technologies. This degradation of the image quality becomes serious when the image is finer. An accurate measurement of the image quality degradation or measuring differences between intended patterns and images distorted by the proximity effects are important for accurate correction of the effects.
Measurement of critical dimension (CD) of the image at a predefined point is important and called CD measurements. With reference to semiconductor integrated circuits, as an example, CD measurement is conventionally done by Critical Dimension Scanning Electron Microscope (CD-SEM) after fabricating the image by the lithography. The CD measurement results are required before fabrication of the image.
To resolve the above mentioned issues related to conventional CD measurement, simulators for analyzing proximity effect have been developed. Using these type of simulators, the degree of image quality degradation or differences between intended patterns and images obtained by the lithography technologies becomes predictable without fabrication. This simulation method still has a problem, wherein these simulators consume huge amounts of computing time for obtaining the lithography image for large area similar to a whole LSI chip. Specifying the CD measurement points is also tedious and time consuming work when the objective area becomes large. Even if CD measurement results are obtained, the amount of data is huge, and it is difficult to understand the result intuitively.
Another issue related to the lithography technology for fine image fabrication is that the obtained image becomes sensitive to the lithography process parameters with progress of image miniaturization. The sensitiveness is referred to as lithography process sensitivity hereafter. The lithography process sensitivity depends on a position of the image and environment of the image. Although knowing lithography process sensitivity at specified points of the image contributes to lithograph technology, no quick method is reported to calculate the lithography process sensitivity.
The conventional CD measurement equipment usage method, such as CD-SEM, as an example, is not efficient. The measurement points are conventionally specified by human engineers, wherein the selected points might not be optimal in the sense of efficient use of measurement equipment. Selecting many measurement points is also difficult for engineers.
In light of the foregoing discussion, a method and system that improves the speed of lithograph simulation and simultaneously improves the efficiency and accuracy of the CD measurement of the image by lithograph technologies is needed. The system should be capable of displaying huge amounts of data obtained by the CD measurement effectively and intuitively. In one aspect, sharing of CD measurement related data between the simulation and the equipment is required for the system to improve efficiency and accuracy of the CD measurement. Although the conventional CD measurement methods focus on visible dimensions of the image, finding potential risk points of the image is essential with reference to the high lithography process sensitivity region. The CD measurement method that uses the lithography simulation should support such requirements.